Vehicles commonly use a pair of crossbars mounted on the roof of the vehicle for mounting various rack assemblies for carrying cargo. Crossbars come in many different sizes and shapes. Yakima sells crossbars having a round cross section. Thule sells crossbars having a square cross section. Auto factory installed crossbars often have more irregular, oblong, elliptical, more aerodynamic shapes.
The diversity in crossbar shapes and sizes has caused a challenge for rack manufacturers to make racks that are adaptable for installation on a wide range of crossbar shapes. Typically, a rack company has to offer a large number of adapters for connecting its racks to different crossbar shapes.